Necrosis is dead plant tissue. On leaves it shows as dry brown or tan tips, margins or spots, or as black, water-soaked patches when disease is involved. It usually follows chlorosis: the tissue first yellows, then dies (Marschner 2012).
In practice
The position and shape of the necrosis help the diagnosis: brown tips on newest mature leaves suggest salt build-up; tan margins on old leaves suggest potassium; spots with yellow halos on new or middle leaves suggest calcium; spots with dark margins and tiny black specks suggest septoria. Necrotic tissue never recovers, so judge any fix by the new growth that follows it (Cockson et al. 2019).
Don't confuse with
- Chlorosis: yellowing that can still recover.
- Senescence: programmed ageing of old leaves.
- Light bleaching: white rather than brown tissue at the canopy top.
Yellowing of normally green plant tissue caused by too little chlorophyll, from nutrient shortage, poor root uptake, pH lockout, light damage, pests or normal ageing.Salt build-up is the accumulation of unused fertiliser and water salts in the growing medium, raising root-zone EC and causing osmotic stress and lockout-type symptoms.A fungal leaf disease caused by Septoria cannabis that produces tan to grey spots with dark margins and yellow halos, starting on lower leaves.The programmed ageing and breakdown of leaves and other tissues, in which the plant recycles nutrients (especially nitrogen) from old leaves to developing flowers and seeds.Light stress is damage when leaves or buds absorb more light than they can use or dissipate, showing as bleached white or yellow tops nearest the fixture.Yellowing of the leaf tissue between the veins while the veins themselves stay green; on old leaves it points to magnesium, on new leaves to iron or manganese.Nutrient burn is browning and crisping of leaf tips, then margins, caused by a root-zone salt concentration (EC) higher than the plant can handle.Any disease that causes discrete dead spots on leaves; on cannabis, mainly fungi such as Septoria cannabis and Alternaria, plus some bacteria.
